The problem is that there is no clear answer. I don't want to buy expensive USB charger and from what I read, the cheap one are not safe (I have one and now I am afraid to use it). I think it will be great if there will be a solution around 2$ - 3$ which is safe to use.

I think it would be great if I hit the $100M powerball lottery!

Google forum search: Use Google Search box in upper right side of this page. Why I like my 2005 Rio Yellow Honda S2000 https://www.youtube.com/watch?v=pWjMvrkUqX0

I thought of using batteries but I want to place all parts in the light switch box so replacing batteries will not be convenient. This is why I thought using the USB charger or other small power supply.

If it has all the right symbols on it then it's passed safety certification.

Cause no one in a third world nation would EVER dream of putting such markings on a device that hadn't actually passed safety inspections!

If you bought the supply from a reputable in-nation dealer (like amazon themselves) it's probably OK.If you bought it from one of the dealers that have sprung up importing things direction from China (and etc), the low prices that you get by bypassing customs probably means that you bypassed real certifications as well. It might be OK, and it might not. Measure it, look at it, make your own judgements.